Optical bar code scanners are being used for a number of different purposes. The best known application for such scanners is in retail checkout operations at supermarkets or mass merchandisers. In retailing environments, the scanner detects a bar code printed on or attached to a product being checked and uses the bar coded information to retrieve the identity and current price of the product from a system memory. The current price and product identity are used primarily to prepare customer receipts and to record transaction totals. The information may also be used for other purposes. For example, the product identity may be used in an inventory control system to track current stocks of the particular product and to automatically reorder the product when the stock falls below a threshold level.
Optical bar code scanners are also used in industrial and/or manufacturing environments for a number of different purposes. One use of a bar code scanner in such an environment is to track incoming or outgoing labelled materials to control the routing of the materials through an automated conveyer system. Another use is to track labelled parts or subassemblies on an assembly line to assure that the proper parts and subassemblies are available at the times and places needed for final assembly of an end product, such as an automobile. There are many other uses for optical bar code scanners in industrial/manufacturing environments.
Because there are fundamental differences between the requirements of retailing and industrial/manufacturing environments, the same type of optical bar code scanner is not necessarily suitable for use in both kinds of environments. In the retailing environment, the product carrying the bar code label can usually be physically positioned relative to the scanner by the checkout stand operator. Therefore, it is not generally considered critical that a checkout scanner be capable of reading bar code labels at widely varying distances from the scanner surface.
In an industrial/manufacturing environment, it is not always possible for a bar code label to be brought within a limited range of distances from a scanner. In such an environment, the item carrying the bar code label may be too heavy or bulky to allow the item to be repositioned solely for the purpose of bringing the bar code label closer to the scanner. In some automated systems, an operator may not be available to reposition an item even where it might be physically feasible to do so.
Different techniques have been adopted in attempts to solve problems encountered in attempting to read bar code labels in an industrial/manufacturing environment. Where the label can't be brought to the scanner, the simplest approach is to bring the scanner to the label by using a hand-held or portable scanner. One problem with this approach is that a label may not located in an easily accessible spot on the item being tracked. Another problem is that an operator must always be available to perform what is basically a mechanical function; namely, maneuvering the hand-held scanner into a position in which the label can be read.
Because hand-held scanners are not well suited for certain industrial/manufacturing applications, attempts have been made to use fixed position scanners for some purposes. Because the distance between the label and the scanner may vary widely in such environments, a fixed position scanner must be designed to have a large depth of field. The "depth of field" of a scanner is the range of distances over which the scanner can successfully read the smallest bar code label allowed by the standards authority for the particular bar code being read. For example, The Uniform Products Code Council issues specifications requiring that UPC (Universal Product Code) labels be no smaller than a predetermined minimum size.
Known scanners employ rotating beam deflectors capable of generating multiple scan lines having different focal lengths; that is, focussed at points at different distances from the scanner. It has been suggested that the depth of field of such a scanner can be maximized by fabricating the beam deflector to focus different scan lines so that the depths of field or focal zones for different scan lines meet but do not overlap substantially. The combined focal zones will provide a continuous depth of field of significant range for the scanner.
One problem with the suggested approach is that the size of the focussed spot must be kept small enough to be able to read the smallest allowable bar code label when that label is detected at the focal point of the scan line having the longest focal length. Since the size of the scanning beam at the beam deflector is fixed, the size of a focussed beam at its focal point is proportional to the focal length of the beam. The sizes of the focussed spots for scan lines having focal lengths shorter than the maximum focal length are necessarily smaller than the size of the focussed spot for the scan line with the longest focal length.
For lines having short focal lengths, the spot size may actually be small in comparison to irregularities of the surface on which the bar code label appears. The surface irregularities may be due to the normal texture of the medium (usually paper) on which the label is printed or may be created by the process of printing the label. Considerable irregularities are produced, for example, when a dot matrix printer is used to produce a bar code label.
If the surface on which a bar code label appears is relatively smooth, a significant part of the optical energy in the impinging light beam is returned or reflected back along the path of the light beam. Returned light eventually reaches and is detected by a photodetector. The photodetector generates an electrical signal having a time varying value dependent primarily on the reflectivity of the surface being crossed by the beam.
If the surface is irregular relative to the size of the beam, however, significant scattering of the optical energy can occur. The portion of the beam returned to the photodetector includes a noise or jitter component as a result of the irregular and intermittent scattering losses. The returned optical signal is degraded by the scattering noise, making it difficult to perform the signal processing operations needed to locate and correctly decode the bar code in the stream of electrical signals produced by the photodetector.